Lead chalcogenide nanoparticles are a family of IV-VI nanoparticles that are of particular interest due to their size tunable optical and emission properties at wavelengths between 1 micron and 3 microns for a variety of applications including, for example, optoelectronics, thermoelectric solid state cooling and power generation, infrared imaging as well as biological applications. In addition, conventional lead chalcogenide semiconductor powders (micron scale) are used extensively in the production of thermo-electric devices, and nanostructured analogs of these materials have been suggested to have potential benefits in performance.
Several methods for the preparation of lead chalcogenide semiconductor quantum dots currently exist, but each has significant deficiencies that have limited the development technologies based on these materials. High temperature growth of lead chalcogenide semiconductor nanocrystals is performed in borosilicate glasses by adding salts of lead and Group VI elements to a glass forming matrix and then heating the mixture above its melting point and homogeneously dissolving the salts of lead and the Group VI elements. The molten mixture is then cooled to a temperature at which the lead cation, Pb+2 and the chalcogenide anion, E−2 become supersaturated and nanocrystals of the lead chalcogenide (hereinafter PbE) nucleate and grow throughout the glass matrix. Further annealing of the samples over periods of days to weeks with careful adjustments in temperature and time of exposure can be used to coarsely adjust nanocrystal size.
The nanocrystals that are formed using this prior art procedure are of good crystal quality and can display sharp optical features in absorption (although emission is usually very poor). Unfortunately, these prior art nanocrystals are trapped in the glass matrix and cannot be extracted for further purification or modification. Moreover, the presence of the insulating glass matrix precludes electrical contact to the nanocrystals, which greatly limits their use in electronic applications.
A second class of synthetic procedures for the production of IV-VI nanocrystals, which occurs at, or near, nominal room temperature, uses the precipitation of PbE salts in an aqueous solution containing at least one surfactant or a mixture of water, an organic solvent and a surfactant. Alternatively these “room temperature preparations” employ a lead salt and use a gaseous reactant for the. chalcogenide source (e.g., H2S, H2Se, or H2Te). In all cases, the room temperature syntheses have proven to be unsatisfactory due to relatively poor control of particle size and poor crystallinity of the resulting particles. Furthermore, the prior art methods that employ gas phase reactants are undesirable due to the extreme toxicity of these reactants and significant cost included in their safe handling and disposal.
A third class of techniques for the preparation of lead chalcogenide nanocrystals (quantum dots) involves gas phase condensation of the evaporated or laser ablated material. These gas phase techniques provide nanocrystals of high purity and crystallinity, but lack the ability to produce monodisperse samples at the levels of 20% standard deviation in size or below in the raw stream. As the gas phase techniques generally produce tens of micrograms to a few milligrams further separation of the material is not practical.
In view of the above drawbacks with prior art synthetic procedures for the preparation of lead chalcogenides and other IV-VI semiconductor nanoparticles, there is a need for providing a new and improved synthetic procedure for preparing IV-VI semiconductor nanoparticles which permits control of the nanoparticle composition, size, shape and surface derivatization, while maintaining high materials yield and allows scale up to large quantities. By controlling the aforementioned features of the semiconductor nanoparticles, the electronic and chemical properties of the nanoparticles can be optimized.